1. Field of the Invention
This invention relates to a system for sputter coating.
2. Description of the Prior Art
Sputtering processes are widely used for the deposition of thin films of materials onto various substrates. In general, the process takes place within a vacuum chamber in which a small quantity of ionisable process gas, for example argon, is present. At appropriate process gas pressures a plasma may be produced through ionisation of the gas by well known means, for example the application of a high voltage between two electrodes within the chamber. A target material, which may itself form the negative (cathode) electrode, is bombarded by positive plasma ions and if the ion bombardment is of sufficient energy target atoms are ejected from the target surface into the vacuum. A substrate placed within the vacuum system, usually with line of sight to and in proximity to the target surface being bombarded, may then be coated by the released target material.
A simple plasma sputtering system, comprising for example of two metal plates separated at an appropriate distance and with a suitable DC voltage between them, is only efficient or useful under a narrow range of process conditions and therefore limited in its application. The evolution of sputtering technology has greatly improved upon such simple systems in the drive to achieve higher deposition rates, better uniformity and properties of deposited films, and wider ranges of materials that can be sputtered. Thus it is well known that AC voltages, usually at radio frequencies (RF) and typically 13.56 MHz can be used, for example to allow the sputtering of insulators, and that magnetic fields can be used to confine or direct plasma electrons, for example to locally increase the plasma energy at the target to enhance sputtering rates. It should be noted that, in general, the achievement of higher deposition rates is a primary commercial goal for sputter systems.
As an example, a circular “magnetron” sputter target assembly has a torus shaped magnetic field penetrating the target material surface to confine plasma electrons and induce a far higher local ionisation level (or ‘plasma density’) than would otherwise be possible. This allows high sputtering rates to be achieved at low gas pressures, typically 1×10−3 to 7×10−3 torr, resulting in high material deposition rates onto substrates and high quality of the deposited thin films. As a result, sputter deposition apparatus using magnetron based designs are extensively used e.g. in the semiconductor and opto-electronic industries.
A variation of the sputter process is reactive sputtering, wherein a process gas or a component of a process gas mixture reacts with the sputtered target material or the deposited thin film to produce a compound material. As an example, an aluminium target may be sputtered under appropriate conditions in a plasma struck in a gas mixture of argon and oxygen to deposit an aluminium oxide film.
To further increase deposition rates and system capability, and overcome some of the limitations imposed by magnetron sputtering systems, it is known that a plasma density of 1011 cm−3 or more, hereinafter a “high density plasma”, can be produced remotely from and independently of the target and then directed to its vicinity by electric and/or magnetic fields.
The major change resulting from using remotely generated plasma is that the sputter target assembly is not required to produce, sustain or contribute energy to the high density plasma. This permits the elimination of the toroidal magnetic field used in the magnetron designs with the result that with the remotely generated plasma guided to the target surface sputtering takes place over the whole target surface, not just the ring of material within the torus.
Effectively the process advantages of the magnetron design are retained whilst a major disadvantage is eliminated. For an appropriately designed and operated system, the plasma density delivered to the target surface is comparable to or greater than that which would be generated in the localised torus of the magnetron design. Since all areas of the target therefore sputter material at the same high rate as is achieved only locally in the magnetron design, the overall deposition rate that may be achieved from the target is greatly increased, typically by a factor of 3 to 5 over the magnetron based system, substantially more when compared to the simple DC or AC powered ‘parallel plate’ based sputter systems.
A variety of techniques are known that may be used to generate remote, high density plasmas, as summarised by Popov in ‘High Density Plasma Sources’ (1995). For example the electron cyclotron resonance (ECR) phenomena may be used to produce a plasma by coupling a microwave source with a strong magnetic field in vacuum.
As a further example high density plasma waves may be generated by the use of an external antenna powered with 13.56 MHz radio frequency signal, as shown in original papers by Boswell and subsequently Chen. These have the advantage of using lower magnetic field strengths compared with ECR, but require careful antenna and magnetic field design to ensure the efficient production and propagation of the ‘helicon wave’ electrons which are used to generate the high plasma densities.
A further, more efficient plasma wave source is used in a sputter deposition system invented by Thwaites (United Kingdom Patent No. 2 343 992). This utilises a helically wound coil antenna in conjunction with non-linear magnetic fields to both produce a high density plasma and to direct this to a target surface out of line of sight of the plasma source. The plasma source has the advantages of a simpler, more robust antenna and magnetic field design than the ‘helicon’ systems and is found to be more efficient in practice.
A limitation of all the disclosed sputter systems is that the target material is ejected in a limited angular arc, therefore limiting the size of the coating area in which substrates may be placed. The angular arc limitation requires that large and therefore expensive vacuum systems, often with many sputter target assemblies, be assembled to efficiently coat large areas and/or large numbers of substrates.
An additional limitation of the disclosed sputter systems is the limited dimensions of the high intensity sputter region that thereby restrict the deposition rate that can be achieved and/or limit the number and size of substrates that may be coated. For example circular magnetron sources sputter from only the magnetic torus, representing less than 20% of the target area; a 200 mm diameter target therefore delivers no more sputtered material than a 90 mm diameter target would if uniformly sputtered.
The remote plasma systems can sputter from the entire target surface, but require a plasma source size of comparable dimension to the target to be sputtered. This generally limits the maximum commercially realistic target size to less than 300 mm diameter. Additionally, geometric and layout requirements for successful implementation of the systems result in larger target to substrate separations being needed; as the deposition rate decreases proportional to the square of the separation distance, the gains anticipated from the large targets may not be realised in practice.